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That's storage with 1m average head though Roger - tidal flow would change the dynamic wouldn't it. I was involved in some run of tide experiments quite a few years back - using kaplan turbines in a designed venturi to accelerate water and increse velocity pressure. Approx 3m diameter collection to an orifice reducing to 1m dia containing the turbine over approx 3m length would generate about 50kW from (I think) 1m/s average tidal velocity.

Many locations would have significantly greater than 2m tidal range which would radically reduce the collection area required also.

Don't figures from potential schemes like the Severn barrage suggest the capacity would equal to something like Drax - about 5% of UK generation ?

OMS,
I thought about the flow, but it seemed to me that the kinetic energy of the flow just becomes (less losses) the potential energy due to the increased water level.
In run of river systems the available power is still a certain mass of water flow past a point with a certain velocity. A tidal barage is the same only cyclic and possibly reversible.
I tried to find more power, but the limit seems to be the slow cycle.

OK Roger, yes, I guess all the flow converts to head for storage of energy anyway - so either we use the flow or we build up the height and use that instead. good point.

I guess the Severn barrage will see massive tidal range (>14m) and a fair old mass of water moving up and downstream - I don't recall the output (I'll look for it) but I recall we were looking at about 5km 2 lagoons (several of them) if the barrage didn't proceed. Th erun of river I mentioned was linked to the edges of the lagoons (as a cheap housing for them) - the venturi was supposed to ensure there was no reduction in velocity downstream (or upstream) of the turbine to avoid deposition of suspended solids etc

Roger
Your original calculation looks sound to me. So if one square kilometer of water at 1 metres deep less the last metre of tide which is lost produces 500kva then just think if the Severn barrage were sited under the old M4 high level road bridge.
We have 25 kilometers beyond it about 2 kilometers wide and average depth of 11-1 metres [ 11 metre tide at Monmouth]. So we could capture 500 cubic kilometers of water and produce 250 MW of power. If we use reversible turbines or water wheels then we capture 500 MW. The siting of the barrage should be reviewed immediately in my view. The barrage should be opened at high and low tides for an hour each time to allow shipping through and forget about the expensive lock; this can be installed later if it proves to be a shipping problem. Lets get on and construct a reliable green energy source and with urgency.
CliveS

The two island nature reserves would have to be relocated while the building work is in progress. Its 2-3 km longer than direct route but may be able to build a stronger barrage stucture with more flexiblity in the design when split the structure into 3 with higher heads/faster flows for less money in selected channel(s).

Perhaps could concentrate all of the flow between two of the hops to minimise environmental flow effects on the welsh coast or the english coast. In effect have one dam (with wildlife provision on the dam causeway along hop 1, to compensate for loss of wildlife habitat on the two islands) and two 5km barrage hops. This may be cheaper and quicker than buiding a single 12-14 km barrage, as can build the 3 sections in parallel to some extent.

May end up creating a slow flow calmer lagoon area behind hop 1 that is even more friendly to wildlife than currently.

Might be even feasible to have just the middle section as the barrage, with two causeway dams, this might act to protect both coasts (with even more space for wildlife) and help make the whole thing cheaper still with only 5 km of expensive barrage structure and turbines.

Hi James
Yes, a barrage that far out would be possibly capture 10 times as much energy and what is more if we built a motorway across the top then we could save billions of pounds on roadworks on the M4 which is already to capacity around Bristol and causing delays to everyone at present.
CliveS

Yes putting a toll motorway (+ tramway?) across would help enormously with the economics of the project. It would give a huge boost to the economy of Weston-super-mare and give Cardiff access to new pool of workers, so they can expand the economy of Cardiff without having to expand the city boundaries. It would generate GW's of electricity. It would help protect against spring tide flooding.

This picture shows the Severn Tidal Power Group - 1989 planned barrage profile.

It shows that they had all their turbines concentrated between the two islands any way. I think fitting all the turbines and sluices into the middle section would mean it is much cheaper to build than their plan and much easier to maintain (with big parts moved in and out by motorway)

If the motorway runs at higher level in the middle between the two islands the big ships can get through somewhere in the middle as well. Choice is between a gate that lowers like the thames barrier or a lock system. I suppose it depends on the amount of ship traffic that is expected, and whether it can all be scheduled at times when energy generation has stopped.

Small coastal boat traffic could run under the motorway at each end in short canal tunnels running above the normal water level with a pumped lock or boat lift at each end (Plenty of power for this after all).

To protect the structure against exceptional storm surges could design it such that water just overtops the causeway sections like a weir, just below the normal level of the motorway.

They are not particulary pristine when looking at the google satellite views, they seem to have a lot of second world war remains on them.

There you are Clive you have your outline plan now. Now we need to find an architect and some civil engineers to do all the detailed designs, calculations and environmental impact assessments. Persuade the government to give planning permission and allow compulsory purchase orders on the islands etc. Then form a consortium and raise X billion to build it.

Being an engineer I don't really think of things like creating jobs for influential politicians so they can supplement their salaries. I would prefer to spend money on engineering talent. The telegraphs scepticism sums up what a lot of people think I expect.

The Telegraphs main complaint is that the tidal range will be reduced harming wildlife so this project is a non-starter.

Won't this depend on how the turbine flow is designed? The barrage will be designed to hold back the water when the tide is going out, to create the head difference. If the water can exit quickly enough through the turbines following the delay, perhaps it can catch up with the outgoing tide, then block the return tide from returning imediately by creating a head difference the other way in the transistion to high tide.

If the structure is sufficiently strong all the water in both directions can be passed through gated turbines.

If the phase of the tide can be delayed and the waveform manipuated, I would have thought it would be possible to keep a large tidal range within the barraged waters for much of the year and so keep most of the mudflats healthy for wildlife.

How achieveable this probably depends on how strong the barrage can be made, and to what extent the water flows can be rapidly ramped and controlled through the turbines / exit gates whilst generating enough of power to make the project work financially.

I assume the design would either have to increase the maximum upper limit to the collective electrical power output of the turbines and grid connections (to over specify them) or to choose when in the tidal cycle to simply dump water fast to catch up with the outgoing tide/incoming tide?

Energy intensive industries that just need intermittent power could relocate and attach close to the grid export points to save unnecessary grid infrastructure spending. The pottery and brick making industries are potential candidates for this.

The intermittent but cyclical power is a problem I was also considering.

If we take the average power available to be 500MW and noting that this will drop to zero 4 times a day (assuming bidirectional turbines) the peak power will have to be somewhat higher but can probably be limited to some extent by flow rate controls, maybe 700MW.

The current grid structure can probably handle variable, but predictable, input of up to 700MW but what will be the additional energy and financial costs of holding spinning reserves of ths level? This means unless a suitable user can be found and possibly relocated to this area almost a complete fosil fuelled thermal station (I think of thermal stations in 1GWe units) will be required to fill in the gaps.

One of the other things he telegraph article does not highlight is that much of the infrastucture spending is for stuff that can last much longer than a gas plant or a nuclear plant.

Nuclear electricity promoters sometimes claim that nuclear power plants can last 60 years, but none has got anywhere near this age without major refurbishment works being necessary (sometimes several times). Nuclear promoters also forget about the billions are wasted at Sellafield, and billions more will be spent there in the decades ahead. This spending will soon act to wipe away all benefits of the short lived subsidised electricity we received whilst the nuclear stations were in operation, and turn into a millstone for us and future generations.

We know gas plants are cheapest at the moment with a 20-40 year life, but that may not be true soon.

Large parts of the barrage and motorway will last on the order of 100-150 years before extensive repairs are needed. Obviously significant parts will require upgrade in the 20-60 time range. What percentage of the overall capital costs this amounts to is not clear.

The comment on the average power output of the tidal barrage is partially valid. There are lots of variables to put in the mix.

One of the interesting variables is how much power comes from the north of the country to the south at this precise moment in time 5.5 GW
(31/10/2012 12:57:00 GMT)

So other things to consider are grid losses and future grid infrastructure investment. There will obviously be important effects on electricity prices when there is mis-match on timing between power generation at the barrage and aggregate power demand on the grid, and this needs to be thought about more.

However this is not really an issue the telegraph can decide conclusively in one short article.

Kiln firing of pottery and brick firing is a cyclical process as long as the order books are full, but for each individual kiln the cycle will be a few days, which doesn't really fit in with the cycling energy output of the barrage.

Could dump part of the barage energy into a high temperature thermal store (analogous to a brick kiln filled with bricks) with a 3-4 hour time constant (at the selected power rating 500 MW, possibly 3000-6000 tonnes of brick with temp rise of 800 degC) located close by. It might be sensible to make this part of a co-located gas station, to avoid unecessary grid infrastucture spending, and effectively convert the barrage+thermal store+gas station into a generating asset with a reasonably stable output.

James Arathoon

(Sorry if I've got the calcs wrong on the thermal store, please correct if I've got the order of magnitude wrong, see below to define minimum limit)

James
The high tide times around the UK maximize at different times. The Thames estuary is high when Bristol is low 6 hours difference. The ideal gap is 3 hours as the grid can get tidal generation from alternative sites at say Exmouth estuary just as an example as the Severn is changing over[high or low]. If the UK government gets its act together we can probably produce at least 75% of the power we need tidally and get a scenic coastal highway at the same time. Roger has worked out that for every square kilometer of estuary we barrage, we can get 1 MW of power over the 24 hour day for each [n-1] metres of rise and fall of tide.
CliveS

Thanks James
Will try to attend in Cardiff as I am developing a design for a new floating water wheel which will only uses the top 1.5 metres of the tide. Environmentally much more friendly than the Kaplan and much much cheaper and quicker to install.
CliveS

Tidel wave energi is going to be the "next big thing". And now we are finally going to get a official test center here in Denmark Link removed/P4/Vest/Nyhed.../2012/10/26/174450.htm (the artikel is in Dainsh). I following a a test project here in Denmark, where the tidal "construction" is placed on the ocean bottom. The test results look promising and a full scale model is now on the way.

Hi James and Kalle
Unfortunately I had to cancel my trip to Cardiff as I caught the flu'. Anyway understand that DECC have shelved the Severn barrage as too expensive and are thinking of more wind in the north sea to placate the EU. Nonsense in my view; we do not want any more wind. What we want is reliable predicable power. We must get on and develop a cheaper way of harvesting tidal power. and NOW
CliveS